Vibration testing method and apparatus

ABSTRACT

Vibration test methods and apparatus for qualifying equipment for use in aerospace vehicles. Multimodal vibration environments are produced for closely simulating in-flight vibration environments, to determine the capacity of the equipment to function in accordance with its intended purposes in operational vibration environments.

val-ass SR 1r U p I s5 0 1 yr j I United States X 3 w Myis] 3,686,927Scharton l Aug. 29, 1972 A- m [54] VIBRATION TESTING METHOD AND I3,100,393 8/1963 Bell ..73/7l.6 APPARATUS 2,706,400 4/1955 Unholtz..73/7l.6 Inventor: Terry D. S m i Famlr ..-.73/7I.6

Calif. Primary Examiner-Jerry W. Myracle [73] Assrgnee: Bolt Beranek andNewman Inc., Att0mey G01ove and Kleinberg Cambridge, Mass. [22] Filed:March 24, 1967 [57] I ABSTRACT [21] Appl. No.: 625,774 Vibration testmethods and apparatus for qualifying equipment for use in aerospacevehicles. Multimodal [52] Us CL 73/716 vibration environments areproduced for closely simu- [51 Int.c1..11IIIXIIII111111111111llllIlllllIllllIiiln 3/32 lating ill-flightvibration environmems, to determine 581 Field of Search ..73/71.4-7l.6;the capacity of the equipment to function in 29/455 cordance with itsintended purposes in operational vibration environments.

[56] References Cited 17 Claims 21 Dra m UNITED STATES PATENTS 2390584accelerometer Multimodol Fixture 42 Exciter.

This invention relates to' vibration testing of equipment, and moreparticularly, to methods and apparatus for vibration testing ofequipment required to function in aerospace vibration environments.

Equipment mounted to an aerospace vehicle is subjected to vibrationenvironments generated in the vehicles exterior structure during flight.Such equipment may comprise delicate electronic equipment or otherequipment which is structurally delicate, and it is important thatin-flight vibration environments do not prevent the equipment fromfunctioning in accordance with the requirements imposed upon it.

In various attempts to evaluate the functional integrity of equipment(that is, to determine the capacity of the equipment to function inaccordance with its intended purposes) in operational vibrationenvironments, vibration tests have been devised for qualifying theequipment prior to service. Current practices include a series of testswhere the equipment is mounted to a rigid test fixture which is vibratedalong a single axis at specified high frequencies by means of amechanical shaker or other vibration exciter. The fixture vibrationlevel is controlled for correspondence with predetermined testspecifications, and the functional integrity of the equipment in thistest vibration environment is noted.

In another series of tests which is in general use, the

equipment is mounted to an aerospace structure such as a large sectionof an aerospace vehicle, and the section base is mounted to a rigid testfixture .which is in turn coupled to a mechanical shaker or othervibration exciter. The test vibration level is measured, generally at anequipment mounting position, and controlled to correspondwith'predetermined test specifications. The

functional integrity of the equipment in this testvibra-' tionenvironment is noted.

Both series of tests entail vibration directed in each of threeorthogonal axes, and vibration tests are performed with respect to oneaxis at a time, in an attempt to simulate the randomness of vibrationenvironments experienced in flight.

The prior art vibration test methods and apparatus provide meaningfulresults when utilized to evaluate the functional integrity of equipmentin low frequency vibration fields, such as when a missile is beingraised on its launch pad or when a spacecraft is slowly wobbling inspace. However, such methods and apparatus cannot simulate the type ofvibration fields produced when light, flexible, aerospace structureresponds to excitation containing many frequencies natural to theaerospace structure. Such excitation is characteristic of the acousticand aerodynamic pressures acting upon the aerospace vehicle, forexample, during launch and shortly thereafter.

Current practices in vibration test programs are based to a large degreeon past experience with low frequency vibration conditions whereproblems of fixture resonance are not usually involved. Such practicesencounter serious difficulties when applied to the problem of attemptingto simulate the effects of vibration environments generated in theexterior structure of in-flight aerospace vehicles. These difficultiesare derived from a conscious effort to avoid resonance of the testfixture.

. frequency range of interest during the vibration tests.

In prior art apparatus, for example, conventional fixtures are designedto be as stiff and rigid as possible, in

order that their natural or resonant frequencies are higher than thefrequencies of the applied excitation. Nevertheless, the first bendingor resonant vibration mode of conventional fixtures often occurs withinthe The vibration mode associated with this first resonant frequencyproduces a large increase in the amplitude of thefixture vibration,which is transmitted to the equipment under test and for whichcompensation must be made by utilizing expensive equalization apparatus.The existence of the first bending mode also results in a spatialvariation throughout the fixture vibration field, so that it isdifficult to measure a representative fixture vibration level which canbe related to the test specifications for excitation control.

Furthermore, conventional rigid test fixtures do not satisfactorilysimulate the impedence of typical aerospace mounting configurations.Rigid fixtures provide a coherent excitation source to the equipmentunder test, in contrast to the incoherent excitation source provided bytypical lightweight and flexible vehicle structure, so that thevibration field applied by a rigid fixture to the equipment under testis unrealistic. Vibrations, transmitted by the coherent excitationsource may further increaseequipment vibration by causing portions ofthe equipment under test to resonate, under conditions where equipmentresonances would not occur in response to a realistic, incoherentsource.

In addition, since rigid fixtures are characteristically quite heavy,large mechanical shakers orother vibration exciters are usually requiredfor generating the large forces required to produce the specifiedfixture vibration levels. At high frequencies, the power loss associatedwith these large forces are substantial, and in some cases conventionaltesting is frustrated by power limitations.

In many cases, the use of rigid test fixtures at high frequencies (e.g.,above 50 cycles per second) frustrates excitation, equalization,- andcontrol endeavors, as well as resulting in expensive and unrealistictests. The occurrence of the lower resonances in the rigid testfixtures, in addition to the unrealistically efficient transmission ofvibrations by rigid fixtures, have often rendered the vibration testdata essentially useless. Further, it is not an uncommon occurrence thatthe equipment under test is vibrated to the point of destruction inrigid fixture tests, although similar equipment had previouslyfunctioned properly in-flight under the same vibration levels.

It has become apparent that high frequency vibration testing ofequipment, for aerospace qualification, cannot be adequately performedwith methods and .apparatus adapted for low frequency operation.Attempts to simulate in-flight vibration environments by utilizing stiffand rigid test fixtures to suppress the fixture resonances, areunrealistic in view of the light weight and flexible nature of aerospacevehicle structure. At

the same time, conventional rigid fixture tests operating in thevicinity of the first few fixture resonances provide greatly amplifiedvibrations which are difficult to relate to test specifications.

The present invention provides methods and apparatus for vibrationtesting equipment for aerospace qualification, and includes apparatusfor generating test vibration fields simulating in-flight vibrationfields and for applying the generated fields to the equipment undertest. The functional integrity of the equipment during exposure to agenerated test vibration field is observed, for evaluating thefunctional integrity of the equipment when exposed to in-flightvibration environments. The methods and apparatus are conceptually basedupon a consideration of characteristic vibration fields generated byaerospace vehicle structure when excited by high frequency energysources, such as provided by acoustic and aerodynamic forces acting uponthe aerospace vehicle exterior structure.

Contemporary aerospace vehicle structure is typically lightweight andflexible. For example, a launch vehicle skin or a space craft shroudcommonly includes an assemblage of interconnected, thin, flexiblemembers, such as metal honeycomb or trusses sandwiched between thin,metal panels. The high frequency energy which is applied to the exteriorstructure in flight, contains many frequencies natural to the flexiblemembers included in the exterior structure, and these members respondthereto by vibrating in various resonant vibration modes," or stationarywave patterns describing reverberant vibration. Each vibrating member,which vibrates in a number of modes cor-- responding to the number ofresonances excited therein by the multi-frequency energy, isadditionally affected by the reverberant vibrations of the othermembers. The reverberant vibrations diffuse or spread throughout thecomplex of interconnected flexible members comprising the exteriorstructure. The vibration field generated by the exterior structure andwhich is applied to equipment coupled to the structure, ischaracteristically reverberant and diffuse, resulting from thesimultaneous excitation of a large number of resonant modes.Accordingly, these vibration characteristics will hereinafter be termedmultimodal.-

The test methods of the present invention include the generating ofmultimodal vibration fields which are applied to the equipment undertest, and which simulate in-flight multimodal vibration environmentsapplied to the equipment in actual service. Oscillatory energy isapplied to a novel type of test fixture, which is adapted to respond tothe applied energy by vibrating in a plurality of resonant modes,generating a multimodal vibration field which may be controlled inaccordance with predetermined test specifications. The equipment undertest, which is coupled to the fixture, responds to the generatedmultimodal vibration field in much the same manner as it would respondto its in-flight vibration environment, and the functional integrity ofthe equipment during the test is observed.

The generation of multimodal vibration fields which simulate in-flightvibration environments, is accomplished by utilizing apparatus coupledbetween at least one source of oscillatory energy and the equipmentunder test. Such apparatus is adapted to have many vibration resonancesin any test frequency bandwidth, and excitation in any frequency bandexcites a large number of vibration modes, resulting in the generationof a reverberant and diffuse (or multimodal) vibration field which isapplied to the equipment under test. Such apparatus, which can be madeto vibrate in many modes, will hereinafter be termed a multimodalfixture.

For example, a multimodal fixture may comprise a flexible member, suchas a thin walled, flexible cylinder, or a simple, flexible plate orpanel. Rather than operating below or in the vicinity of its first fewresonances, the panel (for example) vibrates in a plurality of resonantmodes when excited by a band of high frequency noise.

Although the spatial variation in the response of a prior art rigidfixture vibrating in a single mode is quite large, the vibration fieldof a flexible panel responding in many modes as taught in the presentinvention, is generally quite uniform throughout the panel. In fact, thespatial uniformity in the mean square acceleration of the panel isproportional to the number of structural modes excited.

If a flexible panel were excited only at a single point, the spatialuniformity of the panel response would be partially destroyed when thepanel is loaded by the equipment under test. However, unlike a rigidfixture, the vibration field exhibited by a multimodal structure ischaracteristically insensitive to the details of the applied excitation,so that multipoint excitation can be utilized to increase the spatialuniformity of the loaded panel. For example, a plurality of smallexciters or shakers (not necessarily in phase) can be employed to excitethe panel at various positions thereon, instead of utilizing a singlelarge shaker customary in prior art apparatus.

Another way of preserving spatial uniformity of multimodal response isto couple at least one flexible truss framework to the panel at a numberof points, and to excite the frameworks by at least one excitationsource.

As the fixture configurations became more structurally complex by thecoupling of additional flexible members to the panel, the compositesystem becomes more greatly enriched in its modal response within anymeasurement bandwidth. Indeed, the interactions of many flexiblesystems, each of which exhibit vibration modes corresponding toresonances in a particular frequency bandwidth, contribute to the totalnumber of modes exhibited by the composite system. For example, thetotal number of modes exhibited by a panel with attached flexible beamsis approximately equal to the sum of the number of modes exhibited bythe panel individually and the number of modes of each of the individualbeams.

Since the number of modes of component systems are additive to providethe composite system modal response, it follows that the more componentshaving resonant frequencies within a specified bandwidth, the greaterwill be the number of vibration modes exhibited by the fixture withinthe bandwidth, or the greater will be the fixture modal density."Furthermore, the greater the modal density of each of the componentsystems, the greater will be the fixture modal density.

The modal densities of component members are strongly influenced bytheir geometric characteristics. For example, the modal density of apanel is directly proportional to its area and inversely proportional toits thickness, while the modal density of a beam is directlyproportional to its length and inversely proportional to the square rootof its thickness. Accordingly, the fixture configurations may be:comprised of complex assemblages of long, narrow, flexible beam'membersand thin, flexible panel or plate members.

For example, the preferred ifixture embodiment is comprisedof a complexassemblage of flexible memaerospace mounting configurations are muchbetter simulated in'multimodal'fixtures than in conventional rigidfixtures. The coherent excitation sources provided by the conventionalrigid test fixtures severely overtest the equipment which, in "service,are attached to lightweight structure and subjected to incoherentexcitation sources. The ability of multimodal fixturesto' simulateimpedances of aerospace mounting configurations results in realisticcorrelation to vibration test specifications.

Vibration test specifications are commonly determined from in-flightvibration measurements at structural interfaces, or equipmentmountingpositions. The formulation of these specifications for conventionalvibration tests is complicated by'problems inherent to the rigid testfixtures, involving unrealistic fixture impedances, fixture resonancesin the vicinity of the first vibration mode, and in some cases powerlimitations incident to heavy fixtures, as discussed earlier. Each ofthese problems is avoided by the provision of lightweight, multimodalfixtures taught by the present invention.

The practice of formulating vibration test specifications from interfacemeasurements presents a further difficulty concerning the relating oftest vibrations to in-flight vibrations. Interface vibrationmeasurementson typical aerospace structure may be quite sensitive to the details ofthe attached equipment. It is often difficult, for example, to formulatea meaningful test specification level for particular equipmentfrominflight interface vibration measurements performed with differentequipment. Such problems can be avoided when multimodal fixtures areutilized in the vibration tests, by relating the fixture multimodalvibration level to the multimodal vibration level of the in-flightaerospace vehicle structure, whichisirather insensitive to the detailsof the attached equipment. For example, vibration test specificationsmay be determined from in-flight measurements on aerospace multimodalstructure such as the vehicle skin or spacecraftshroud. This in-flightmultimodal vibration level can then be utilized to set the multimodalvibration test levels on. the multimodal fixtures, measured at a pointsomewhat remote from the fixture-equipment interface.

In addition to multimodal fixtures which simulate typical vibrationfields produced by contemporary aerospace construction, othermultimodal: test fixtures may be designed for'simulating specificvibration fields produced by particular aerospace mounting structure.Utilization of such fixtures, produced onan ad hoc basis upondevelopment of specific vehicle structure,

generates vibration fields which simulate in-flight :environments betterthan'thosexgenerated in prior art test apparatus utilizing a largesection of a particular aerospace vehicle (orthe complete-vehicleoramodel thereof) coupled to a conventional :rigid fixture. ln

- these prior art tests, furthermore, the combination of the heavyfixture and the large vehicle structure present problems of powerdeficiencies and --vehicle structure support. In addition, theprovidingof the par- 'ticular vehicle structure for mountingzthe equipment undertest in veryexpensive, and complex size-effect problems are presented ifa scaledldown model-of the vehicle or a section of the vehicleisusedxOfcourse, excitation specification control problems (includingfirstfixture resonance and fixture coherence) are still present,althoughthe effects of these latter problems upon the equipment undertest are somewhat lessened because of the multimodal nature of theaerospace mounting structure'Further, theproblem of damaging thestructure in the vicinity of the rigidfixture, particularly at the firstfixture resonance, is present.

Inplace of the current .practicedescribed.-above,a multimodal fixturehaving vibration characteristics simulating particular aerospacestructure :can beiprovided. Such a fixture includes a relativelysmall":section of an aerospace vehicle (herebydefined to.include amemberwhich structurally simulates a section of an aerospace vehicle), towhich additional multimodalelements are attached. The modal densityinherent to the section is thereupon enriched and .the

multimodal vibration field generated when the fixture.

isexcited is characterized by the increased modal density of thesection. Although excitation may be provided by one large shaker,theutilization of a number of smaller shakers is preferred forincreasing the. spatial uniformity of the fixtureresponse.

ments (i.e., the original section .andthe added members),,the modaldensity of the sectionhas thus been enriched, to simulate the modaldensity oftthecomplete vehicle. The effect of enriching themodal-densityof the small section is to cause attached equipmentto be subjected toatest vibration environment .which very nearly simulates its in-flightvibrationenvironment.

An important advantage of the vibrationtest method of the presentinvention, is related to thediffuse propertyof the multimodal vibrationfieldsgeneratedby multimodal fixtures. Multimodal vibration ischaracteristically quite insensitive to the details ofthetexcitation,-so that the direction'andexact.locationof-theexcitationis relatively unimportant in determiningthe fixture response. Besidespermitting the utilization of mul- .tipoint excitation, as discussedearlier, the :diffuse vibration fields generated by multimodalfixtures:avoid the necessity of performing separate vibration 1 tests along eachof three mutually orthogonal axes,=since' the equipment attached to theexcited multimodal fixture. is

vibrated in all directions simultaneously.

It is an object of the present invention to provide a method ofvibration testing equipment which is to be utilized in a multimodalvibration environment, to determine the functional integrity of theequipment prior to actual service.

It is another object of the present invention to pro vide a method ofvibration testing equipment which is to be utilized in a multimodalvibration environment produced by in-flight excitation of typicalaerospace mounting SIIUCIU1'.

It is a further object of the present invention to provide a method ofvibration testing equipment which is to be utilized in a multimodalvibration environment produced by in-fiight excitation of particularaerospace mounting structure.

It is yet another object of the present invention to provide apparatusfor generating a multimodal vibra tion field which simulates inflightvibration environ ments in aerospace vehicle structure.

It is another object of the present invention to provide apparatus forgenerating a multimodal vibration field when oscillatory energy isapplied thereto, and for applying the generated field to equipment undertest.

It is a still further object of the present invention to provideapparatus for generating a multimodal vibration field simulating anactual vibration field environmental to equipment in service.

It is yet another object of the present invention to provide a vibrationtest fixture having many resonant modes of vibration excited by .atleast one frequency component of applied oscillatory energy.

It is another object of the present invention to provide a vibrationtest fixture for generating a reverberant and diffuse vibration fieldwhich vibrates the equipment under test in all directionssimultaneously.

It is still another object of the present invention to provide avibration test fixture which includes a section of particular multimodalstructure and which further includes means for enriching the modaldensity of the section.

It is a further object of the present invention to provide a vibrationtest fixture which utilizes a section of an aerospace vehicle, forgenerating a vibration field which is characteristic of the completevehicle.

The novel features which are believed to be characteristic of themethods and apparatus of the present invention, together with furtherobjects and advantages thereof, will be better understood from thefollowing description considered in connection with the accom panyingdrawings in which several embodiments of the invention are illustratedby way of example. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only, andare not intended as a definition of the limits of the invention.

FIG. 1 is a part elevation and part block diagram of conventional rigidfixture vibration test apparatus, for vibrating equipment along avertical axis;

FIG. 2 is a part elevation and part block diagram of conventional rigidfixture test apparatus, for vibrating equipment along a horizontal axis;

FIG. 3 is a side view of the rigid fixture of FIG. 1, exhibiting itsfirst resonance;

FIG. 4 is a block diagram of vibration test apparatus according to thethe present invention;

FIG. 5 is a series of representative resonant vibration modes of aflexible beam;

FIG. 6 is a side view of a preferred embodiment of a multimodal fixtureaccording to the present invention;

FIG. 7 is a side view of a first example of the preferred fixtureembodiment shown in FIG. 6;

FIG. 8 including FIGS. 8a and 8b is a side view of a second example of afixture embodiment; and a top view of a component element, respectively;

FIG. 9 is a side view of a third example of a fixture according to thepresent invention;

FIG. 10 is a side view of a fourth example of a fixture according to thepresent invention;

FIG. 11 is a side view of a first alternative embodiment of a multimodalfixture according to the present invention;

FIG. 12 is a side view of a second alternative embodiment of amultimodal fixture according to the present invention;

FIG. 13 is a side view of a third alternative embodiment of a multimodalfixture according to the present invention;

FIG. 14 is a side view of a fourth alternative embodiment of amultimodal fixture according to the present invention;

FIG. 15 is a plan view of a fifth alternative embodiment of a multimodalfixture according to the present invention;

FIG. 16 is a plan view of a sixth alternative embodiment of a multimodalfixtrue according to the present invention;

FIG. 17 is a side view of a seventh alternative embodiment of amultimodal test fixture, utilizing a section of an aerospace vehicle;

FIG. 18a is a side view of a further example of the seventh alternativeembodiment of a multimodal test fixture, utilizing a section of anaerospace vehicle;

FIG. 18b is a perspective view of the multimodal fixture shown in FIG.

FIG. 19 is a perspective view of one example of an embodiment of amultimodaltest fixture, utilizing a section of an aerospace vehicle; and

FIG. 20 is a perspective view of a second example of an embodiment of amultimodal test fixture utilizing a section of an aerospace vehicle.

Turning first to FIG. 1, there is shown prior art apparatus forvibration testing equipment such as a component 10, along a verticalaxis, by conventional techniques utilizing a rigid fixture 12. The rigidfixture 12 is commonly a slab of aluminum or magnesium, generallyseveral inches thick in order that it be as stiff and rigid as possible.The component 10 is mounted to upper surface 14 of the rigid fixture 12,by mounting members 16 attached to both the component 10 and the uppersurface of the rigid fixture 12. Lower surface 18 is rigidly coupled ata centralized location to a shaker or other vibratory exciter 20, forvertically reciprocating the rigid fixture 12. The resulting vibrationsof the upper surface 14 in the vicinity of the component 10 arethereupon transmitted to the component 10 which vibrates in responsethereto.

The fixture vibration level is measured by means of a transducer, suchas an accelerometer 22 having a feeler element 24 engaging the rigidfixture 12 at a representative location on its upper surface 14 (i.e., alocation at which a measured vibration level is assumed to berepresentative of vibration levels throughout the upper surface), andfurther including readout means 26. The vibratory force from the exciter20 is adjusted until the fixture vibration response, measured by theaccelerometer 22, corresponds to predetermined vibration testspecifications. The functional integrity of the component 10 in thistest vibration environment is ob served.

In an alternative test, a component 10' is mounted to aerospacestructure 27, such as an aerospace vehicle, a model of an aerospacevehicle, or a large section of an aerospace vehicle. The aerospacestructure 27 is mounted to the upper surface 14 of the rigid fixture 12by mounting members such as brackets 28, and the component 10' ismounted to the aerospace structure 27 by means of mounting members 16.When the rigid fixture 12 is vibrated by the exciter 20, theaccelerometer 22 measures the level of vibrations influencing thecomponent 10', suchas by engaging an accelerometer feeler element 24'with a mounting memberl6'. The vibration force from the exciter isadjusted so that the accelerometer response corresponds to predeterminedtest specifications, and the functional integrity of the component 10 inthis alternative test vibration environment is observed.

Since the vibration field which the test apparatusattempts to simulateis omnidirectional, it is the general practice to perform additionalvibration tests on the component, where the vibrations are directedalong each of two mutually orthogonal horizontal axes. For

example, the component 10 or the aerospace structure 27 with component10 attached, is removed from the vertical vibration test apparatus ofFIG. 1 and is coupled to the test apparatus of FIG. 2 for vibrating thecomponent along the horizontal axes.

Turning to FIG. 2, the component 10 (or an aerospace structure withcomponent attached, not

shown) is mounted to upper surface 30 of a conventional rigid fixture32. The rigid fixture 32 is commonly a thick slab or ring of aluminum ormagnesium, and has a substantially flat lower surface 34. The rigidfixture 32 is slide-ably supported by a table 36 which has asubstantially flat, smooth, upper surface 38, customarily provided by ahighly ground surface of a marble slab. A fluid bearing is situatedbetween the fixture lower surface 34 and the' table upper surface 38, topermit horizontal movement of the rigid fixture 32 over the stationarytable upper surface 38. The fluid bearing 40 may be a film of either aliquid or gaseous substance, and means (not shown) are usually providedfor maintaining the fluid bearing 40. A shaker or other vibrationexciter 41 is coupled along a first horizontal axis of the rigid fixture32, for producing horizontal vibrations in the rigid fixture along thisfirst horizontal axis, which vibrations are transmitted to the component10. The fixture vibration response is measured at a representativelocation, in the same manner as in FIG. 1, for controlling excitation inaccordance with the predetermined test specifications, and thefunctional integrity of the component 10 is observed.

After the component 10 is vibration tested along the first horizontalaxis, the exciter 41 is repositioned with respect to the rigid fixture32 for producing vibrations along a second horizontal axis orthogonal tothe first horizontal axis. The component 10 is thereupon vibrationtested along the second horizontal axis, in accordance with the methodpreviously described.

In the prior art apparatus, the conventional fixtures are designed to beas stiff and rigid as possible, in order that all of their resonantfrequencies are higher than the frequencies of the applied excitation.Nevertheless, the first resonance of conventional fixtures often occurswithin the frequency. range of interest during vibration tests,resulting in large increases in fixture acceleration as well as aspatial variation throughout the fixture vibration field.

In FIG. 3, for example, the rigid fixture 12 of the apparatus shown inFIG. 1, is shown exhibiting the bending mode which is excited by thefixtures first resonant frequency. The increase in vibration amplitudeand the spatial variation throughout the upper surface 14 of the rigidfixture 12 is indicated from the drawing.

Turning to FIG. 4, there is shown vibration test apparatus according tothe present invention. Instead of utilizing a rigid .test fixture tosuppress the fixture resonances, as in the prior art test apparatus, theapparatus of the present invention includes-a multimodal test fixture42, which generates a reverberant and diffuse vibration field andapplies the generated field to an attached component 44, when excited byoscillatory energy from at least one coupled vibration exciter 46. Thecomponent 44 is coupled to the multimodal fixture 42 by mounting members48 which simulate mounting configurations of the component in anaerospace vehicle.

The fixture vibration level is measured ata representative position onthe multimodal fixture 42, by means of a transducer such as anaccelerometer 50 having a feeler element 52 and readout means 54. Theexciter 46 is controlled to provide a measured vibration level whichcorresponds to the test specifications, and the functional integrity ofthe component 44 is observed. Although the vibration measurements may beperformed at or near one of the component mounting members 48, it ispreferred that the measurements be performed at a location on themultimodal fixture 42 somewhat distant from the component 44. It isfurther preferred that the test specifications be formulated fromin-flight vibration measurements on the vehicle multimodal structure,instead of the current practice of formulating the vibration testspecifications from inflight vibration measurements at equipmentmounting positions. The multimodal vibration test level can thereupon becontrolled for correspondence with the preferred test specifications sothat both the character and the level of the vibration field generatedby the multimodal fixture 42 and applied to the component 44 simulatesthe character and level of the in-flight vibration field.

The multimodal vibration field generated by the multimodal fixture 42can be better described with reference to FIG. 5, which illustrates anumber of bending or vibration modes associated with different resonantfrequencies of a thin, flexible beam 56 whose ends are supported byhinges 58, 60. The first pattern shown beneath the beam 56 is arepresentation of the mode experienced by the beam when excited byenergy which includes the beams first resonant frequency.

in the fixture vibration level This first vibration mode is a stationarypattern having nodes at the beam ends and an antinode at its center; thebeam vibrates through a large amplitude at its center. Other vibrationmodes are represented in FIG. 5, in increasing order of resonantfrequency, each showing the beam to vibrate in sinusoidal stationarypatterns. As the order of the resonant frequency is increased, thewavelength of the corresponding vibration mode exhibited by the beam 56is decreased; the amplitude of vibration is correspondingly decreased.Although all vibration modes describe stationary patterns, therepresentation of modes having an order higher than five are shown inFIG. as simple sine waves. Furthermore, the beam 56 has a very largenumber of modes and FIG. 5 shows a gap in their representation after the12th mode until an n'" mode is reached.

When the beam 56 is excited by energy containing many resonantfrequencies, the beam exhibits vibration in each of its correspondingmodes, the vibration characteristics of each mode being substantiallyindependent of those of the other excited modes. As the applied resonantfrequencies become increasingly higher than the fundamental or firstresonant frequency, the beams vibration pattern becomes substantiallyindependent of its end conditions.

A thin plate or panel may be regarded as a two dimensional beam, andresonant vibrations may be induced in the panel by analogy to resonantvibrations of the beam. Furthermore, if the energy applied to the beam,or to a panel, includes many higher order resonant frequencies, and nonebelow the th resonant frequency, for example, the vibration response ofthe beam will be quite uniform throughout its area. Multimodal fixturesaccording to the present invention generate vibration responses havingsuch spatial uniformity, by having the capacity to exhibit many higherorder bending modes within any test frequency band of interest, withoutexhibiting the first few modes which are characterized by highamplitudes and low spatial uniformity. It should be noted that at thesehigher frequencies, multimodal fixtures have a large number of resonantmodes close together in frequency in any bandwidth, so that theintroduction of a small amount of damping causes the modes to overlap,thereby smoothing the fixture response with respect to frequency.

Multimodal fixtures may be comprised of a combination of flexiblemembers, each exhibiting a resonant mode when excited by a particularfrequency of applied oscillatory energy. The fixture vibrates in manymodes, since the total vibration of the composite system approximatesthe sum of the component vibrations. However, the number of modes of thefixture vibration can be increased if each of the component members hasmany resonant modes and the applied oscillatory energy includes manyresonant frequencies. Accordingly, vibration tests may be performedutilizing multimodal fixtures where the oscillatory energy contains aparticular frequency bandwidth, and may be repeated for differentbandwidths. Alternatively, tests may be performed by exciting thefixture with oscillatory energy of a single frequency, or withoscillatory energy of time varying frequency such as a sine sweep, andmeasuring the fixture response as a function of frequency.

Turning to FIG. 6, a side view of a preferred embodiment of a multimodalfixture according to the present invention is shown. A system 62 offlexible members is sandwiched between a first flexible plate or panel64 and a second flexible plate or panel 66. The first panel 64 isadapted to be coupled to a vibration exciter, or a plurality ofvibration exciters, and the total flexible structure generates amultimodal vibration field in the second panel 66, which is adapted tobe coupled to the equipment under test and to apply the multimodalvibration field to the equipment. The flexible panels 64, 66 may beeither flat or curved, such as a portion of a cylinder.

The combination of the flexible panels 64, 66 and the system 62 offlexible members provides a fixture of great modal density. It is thesystem 62 of flexible members, in particular, which makes the greatestcontribution to the modal density of the fixture.

The system 62 of flexible members may include an assemblage of flexiblebeam members, and the beam members may have any shape with respect toboth their lengths and cross sections, as long as flexibility ispreserved. A side view of one possible beam configuration is shown inFIG. 7, in which an assemblage of flexible beams 68 is coupled betweenthe first and second panels 64, 66. Although the beams 68 are shown inFIG. 7 as being arranged in a particular orderly fashion, other beamconfigurations are possible which include other orderly arrangements aswell as random arrangements.

Such beam configurations further include assemblages of wirework, suchas a wire network 70 as shown in FIG. 8. A side view of a possible wirenetwork configuration is shown in FIG. 8a, while a plan view of apossible type of wire network 70 is shown in FIG. 8b. Other types ofwirework may be utilized in the flexible system 62, such as anassemblage of coiled wire 72 shown in FIG. 9.

Other examples of the preferred embodiment of FIG. 6, utilizingdifierent configurations of the system 62 of flexible members, arepossible. For example, in FIG. 10 there is shown an assemblage of thin,flexible plates 74 coupled between the first and second panels 64, 66.The plates 74 may be either orderly or randomly arranged.

In FIG. 11, there is shown a first alternative embodiment of amultimodal fixture according to the present invention, in which aplurality of beams 76 is coupled to a first surface 78 of a flexiblepanel 80. The uncoupled ends of the beam 76 are interconnected, forexample at a central location 82, forming a truss network 84. Excitationmay be supplied to the truss network 84 either by a single vibrationexciter (not shown) coupled to the beam interconnection location 82, orby a plurality of smaller vibration exciters positioned throughout thetruss network 84. A second surface 86 of the panel is adapted to becoupled to the equipment under test (not shown), for applying themultimodal vibration field to the equipment.

Similarly, a plurality of truss networks may be provided, such as a dualtruss network included in a second alternative embodiment of amultimodal fixture, shown in FIG. 12. Two truss networks 88, 90 areprovided, each comprised of a plurality of beams 92. In this example,each truss network 88, 90 has a point of interconnection 94, 96,respectively, to which vibration l3 exciters may be coupled, althoughexcitersmay be coupled to the truss networks at other locations. Themultimodal vibration field is applied to the equipment under test whenthe equipment is mounted to a flexible panel 98 coupled to the trussnetworks 88, 90.

A third alternativeembodiment of a multimodal fixture is, shown in FIG.13, and comprises a simple, flexible panel 100. In normal operation, theequipment under test is mounted to a first surface 102 of the panel anda plurality of vibration exciters 104 are coupled to a second surface106 of the panel for applying multifrequency energy thereto.

As shown in FIG. 14, a fourth alternative embodiment of a multimodalfixture utilizes an assemblage 108 of flexible members 110, such asbeams or plates, coupled to a first surface 1 12 of aflexible panel 114.A plurality of vibration exciters 116 arecoupled to a second surface 118of the panel 114, and the equipment under test is mounted to this secondsurface.

In FIG. 15, there is shown a plan view of a fifth alternative embodimentof a multimodal fixture which has particular utility for systems tests,i.e., for testing the functional integrity of a complete. system such asa spacecraft. A first flexible cylinder 120 is provided, which may beseveral feet in diameter. A second flexible cylinder 122 is positionedconcentrically within the first flexible cylinder 120, and a system 124of flexible members (for example, beams or plates, as discussed inconjunction with the various configurations of the preferred embodimentof FIGS. 6 through is coupled therebetween. The equipment to be testedis coupled to the second cylinder, for example, by means of mountingbrackets 126, and excitation is applied to the first cylinder 120 bymeans of a plurality of vibration exciters 128.

A sixth alternative embodiment of a multimodal fixture, shown in FIG.16, also has particular utility for systems vibration tests. A trussnetwork 130 of flexible members 132 (for example, beams or plates) iscoupled to the interior surface 134 of a cylinder 136. The cylinder 136is excited by a plurality of vibration exciters 138 which may bepositioned on the exterior surface 140 of the flexible cylinder 136. Theequipment to be tested may be mounted to the flexible cylinder 136, forexample, by means of mounting brackets 142.

Other embodiments of multimodal fixtures accord ing to the presentinvention may be designed for simulating specific vibration fieldsproduced by a particular aerospace vehicle. Such embodiments include asection of the aerospace vehicle, as defined earlier,and at least oneassemblage of flexible structure (or multiniodal elements) is coupled tothe section in order to enrich its modal density.

For example, a seventh alternative embodiment of a multimodal fixture isshown in FIG. 17, in which a plurality of flexible members 144 (such asbeams or plates) are coupled to the boundaries of a section 146 of aparticular aerospace vehicle. Excitation may be applied to a firstsurface 148 of the section, and the equipment under test may be mountedto a second surface 150 of the section.

As a further example of the seventh alternative embodiment, shown inFIGS. 18a and'l8b, a section 152 of a particular aerospace vehicle ismodally enriched by the attachment thereon of rolls of wire network 154at the boundaries 156 of the section.

Besides coupling multimodal elements ,to the sections boundaries, themodal density of a particular aerospace section maybe enriched bycoupling multimodal elements to a surface of the section, in similarfashion to the preferred fixture embodiment of FIGS. 6 through 10, andthe alternative embodiments of FIGS. 11,12, 14, 15 and 16.

For example, in FIG. 19, there is shown an example.

of an embodiment of a multimodal fixture in which a section 158 of aparticular aerospace vehicle is utilized. A truss network 160 ofinterconnected beams 162 is coupled to one surface of the section 158,in. the same manner that, in the first alternative embodiment (FIG. 11),the truss network 84 is coupled to 'the flexible panel 80.

As a further example, the multimodal fixture shown in FIG. 20 comprisesa cylindrical section 1640f a particular aerospace vehicle, and includesatruss network 166 of flexible members 168 (such as beams or plates)coupled to its interior surface 170. This fixture is similar to thesixth alternative embodiment (FIG. 16) in which the flexible cylinder136 is replaced by the cylindrical aerospace section.

Thus, there has been described a method for vibration testing equipmentrequired to function in vibration environments occurring in aerospacestructure. The method includes the generation of multimodal vibrationfields which are applied to equipment under test, for simulating thevibration environments of the equipment in-flight. Further, variousembodiments of multimodal test fixtures for utilization with the methodof the present invention have been described.

Other embodiments of apparatus to practice the method of the presentinvention, and modification of the embodiments herein presented, may bedeveloped without departing from the essential characteristics thereof.For example, multimodal structures other than those specificallypresented herein, may be incorporated in a multimodal fixture withoutdeparting from the scope of the present invention. Furthermore, the

test method should not be restricted to simulating aerospace vibrationenvironments, but may be utilized to simulate other multimodal vibrationenvironments.

Accordingly, the invention should be limited only by the scope of theclaims appended below.

What is claimed as new is: 1. Apparatus for generating a diffuse,reverberant vibration field and for applying the generated vibrationfield to equipment under test, comprising the combination of:

first means adapted to be coupled to the equipment under test and havingstructure for sustaining therein a diffuse, multi-resonant vibrationfield and for applying said field to the equipment;

second means coupled to said first means and adapted to cause said firstmeans to vibrate simultaneously in a multiplicity of diffuse, resonantvibration modes when excited by applied oscillatory energy; and

means for applying oscillatory energy to said second means and forthereby generating a multimodal vibration field in said first means.

2. Apparatus according to claim 1, above, wherein:

said first means has an inherent modal density and said second meansalters said inherent modal density to create a predetermined modaldensity in said first means, the multimodal vibration field generated inresponse to applied oscillatory energy being characterized by saidpredetermined modal density.

said second means comprising a system of second flexible members, saidsystem being coupled to said first flexible member.

10. Apparatus according to claim 9, above, wherein 3. Apparatus forgenerating a vibration field and for said first flexible member is aflexible plate.

applying the generated field to equipment under test, comprising thecombination of:

first means including a first flexible member adapted to be coupled tothe equipment under test for applying a vibration field generatedtherein to the equipment; second means including a second flexiblemember of substantially similar contour to said first flexible memberand positioned substantially parallel thereto, and a system of thirdflexible members, said system being positioned between said first andsecond flexible members and coupled thereto, said second means beingcoupled to said first means and adapted to cause said first means tovibrate simultaneously in a plurality of vibration modes when excited byapplied oscillatory energy; and

means for applying oscillatory energy to said second means and forthereby generating a multimodal vibration field in said first means. 4.Apparatus according to claim 3, above, wherein said first and secondflexible members are flexible plates.

5. Apparatus according to claim 3, above, wherein said first flexiblemember is a first flexible cylinder, and said second flexible member isa second flexible cylinder concentric to said first cylinder.

6. Apparatus according to claim 3, above, wherein said first flexiblemember is a section of an aerospace vehicle.

7. Apparatus according to claim 3, above, wherein said system of thirdflexible members includes an assemblage of flexible plates.

8. Apparatus according to claim 3, above, wherein said system of thirdflexible members includes an assemblage of flexible beams.

9. Apparatus for generating a vibration field when oscillatory energy isapplied thereto, and for applying the generated field to equipment undertest, comprising the combination of:

first means adapted to be coupled to the equipment under test forapplying a diffuse, reverberant vibration field generated therein to theequipment,

said first means comprising a first flexible member;

and

second means coupled to said first means and adapted to cause said firstmeans to vibrate simultnaeously in a plurality of vibration modes whenexcited by applied oscillatory energy Said apparatus generating amultimodal vibration field in said first means,

11. Apparatus according to claim 9, above, wherein said first flexiblemember is a flexible cylinder.

12. Apparatus according to claim 9, above, wherein said first flexiblemember is a section of an aerospace vel gcle.

Apparatus according to claim 9, above, wherein said system of secondflexible members includes at least one assemblage of flexible beams,each of said assemblages being coupled to said first flexible member.

14. Apparatus according to claim 9, above, wherein said system of secondflexible members includes at least one assemblage of flexible plates,each of said second flexible members being coupled to said firstflexible member.

15. Apparatus for generating a vibration field and for applying thegenerated field to equipment under test, comprising the combination of:

at least one source of oscillatory energy which includes a plurality offrequency components; and

a flexible member coupled to each of said sources and adapted to becoupled to the equipment under test and to respond selectively to saidenergy by vibrating in a plurality of vibration modes excited byparticular frequency components of said energy, for generating amultimodal vibration field in said member when said energy is appliedthereto, and further adapted to be coupled to the equipment under testfor applying the generated field to the equipment.

16. A method of testing equipment in vibration comprising the steps of:

coupling the equipment to a fixture adapted to sustain therein adiffuse, multi-resonant vibration field including a multiplicity ofvibration modes when oscillatory energy is applied thereto; applyingoscillatory energy to said fixture and thereby generating in response tosaid energy a diffuse, multi-resonant vibration field on said fixturewhich is thereupon applied to the equipment; measuring a multi-resonantvibration level at a representative location on said fixture; and

adjusting said applied oscillatory energy to modify said diffuse,multi-resonant vibration field for correspondence thereof topredetermined test specifications, while noting the functional integrityof the equipment.

17. The method according to claim 16, above, wherein said oscillatoryenergy is applied to said fixture by means of a plurality of energysources coupled thereto.

1. Apparatus for generating a diffuse, reverberant vibration field andfor applying the generated vibration field to equipment under test,comprising the combination of: first means adapted to be coupled to theequipment under test and having structure for sustaining therein adiffuse, multiresonant vibration field and for applying said field tothe equipment; second means coupled to said first means and adapted tocause said first means to vibrate simultaneously in a multiplicity ofdiffuse, resonant vibration modes when excited by applied oscillatoryenergy; and means for applying oscillatory energy to said second meansand for thereby generating a multimodal vibration field in said firstmeans.
 2. Apparatus according to claim 1, above, wherein: said firstmeans has an inherent modal density and said second means alters saidinherent modal density to create a predetermined modal density in saidfirst means, the multimodal vibration field generated in response toapplied oscillatory energy being characterized by said predeterminedmodal density.
 3. Apparatus for generating a vibration field and forapplying the generated field to equipment under test, comprising thecombination of: first means including a first flexible member adapted tobe coupled to the equipment under test for applying a vibration fieldgenerated therein to the equipment; second means including a secondflexible member of substantially similar contour to said first flexiblemember and positioned substantially parallel thereto, and a system ofthird flexible members, said system being positioned between said firstand second flexible members and coupled thereto, said second means beingcoupled to said first means and adapted to cause said first means tovibrate simultaneously in a plurality of vibration modes when excited byapplied oscillatory energy; and means for applying oscillatory energy tosaid second means and for thereby generating a multimodal vibrationfield in said first means.
 4. Apparatus according to claim 3, above,wherein said first and second flexible members are flexible plates. 5.Apparatus according to claim 3, above, wherein said first flexiblemember is a first flexible cylinder, and said second flexible member isa second flexible cylinder concentric to said first cylinder. 6.Apparatus according to claim 3, above, wherein said first flexiblemember is a section of an aerospace vehicle.
 7. Apparatus according toclaim 3, above, wherein said system of third flexible members includesan assemblage of flexible plates.
 8. Apparatus according to claim 3,above, wherein said system of third flexible members includes anassemblage of flexible beams.
 9. Apparatus for generating a vibrationfield when oscillatory energy is applied thereto, and for applying thegenerated field to equipment under test, comprising the combination of:first means adapted to be coupled to the equipment under test forapplying a diffuse, reverberant vibration field generated therein to theequipment, said first means comprising a first flexible member; andsecond means coupled to said first means and adapted to cause said firstmeans to vibrate simultnaeously in a plurality of vibration modes whenexcited by applied oscillatory energy, said apparatus generating amultimodal vibration field in said first means, said second meanscomprising a system of second flexible members, said system beingcoupled to said first flexible member.
 10. Apparatus according to claim9, above, wherein said first flexible member is a flexible plate. 11.Apparatus according to claim 9, above, wherein said first flexiblemember is a flexible cylinder.
 12. Apparatus according to claim 9,above, wherein said first flexible member is a section of an aerospacevehicle.
 13. Apparatus according to claim 9, above, wherein said systemof second flexible members includes at least one assemblage of flexiblebeams, each of said assemblages being coupled to said first flexiblemember.
 14. Apparatus according to claim 9, above, wherein said systemof second flexible members includes at least one assemblage of flexibleplates, each of said second flexible members being coupled to said firstflexible member.
 15. Apparatus for generating a vibration field and forapplying the generated field to equipment under test, comprising thecombination of: at least one source of oscillatory energy which includesa plurality of frequency components; and a flexible member coupled toeach of said sources and adapted to be coupled to the equipment undertest and to respond selectively to said energy by vibrating in aplurality of vibration modes excited by particular frequency componentsof said energy, for generating a multimodal vibration field in saidmember when said energy is applied thereto, and further adapted to becoupled to the equipment under test for applying the generated field tothe equipment.
 16. A method of testing equipment in vibration comprisingthe steps of: coupling the equipment to a fixture adapted to sustaintherein a diffuse, multi-resonaNt vibration field including amultiplicity of vibration modes when oscillatory energy is appliedthereto; applying oscillatory energy to said fixture and therebygenerating in response to said energy a diffuse, multi-resonantvibration field on said fixture which is thereupon applied to theequipment; measuring a multi-resonant vibration level at arepresentative location on said fixture; and adjusting said appliedoscillatory energy to modify said diffuse, multi-resonant vibrationfield for correspondence thereof to predetermined test specifications,while noting the functional integrity of the equipment.
 17. The methodaccording to claim 16, above, wherein said oscillatory energy is appliedto said fixture by means of a plurality of energy sources coupledthereto.